Special ceramic compositions are manufactured by sintering ceramic powders. There has been a continual search for sintering processes that will provide a sintered compact that approaches theoretical density at the lowest possible sintering temperatures and in the shortest possible length of time. Two processes, uniaxial hot pressing and hot isostatic pressing ("hipping"), are commonly used to fully densify ceramic compositions.
With uniaxial hot pressing, the powders are compacted in graphite molds that are heated, for example, by a high frequency furnace. See U.S. Pat. No. 4,657,877. The hot pressing process has a number of drawbacks: Hot pressing is a labor intensive and time consuming process, and complex shapes are not easily formed. For example, it is not easy to form shapes with a central opening therein. Finally, the uniaxial pressure applied during uniaxial hot pressing affects the grain structure of the sintered product such that some of the properties are anisotropic.
With hot isostatic pressing, the powders are usually precompacted and then the surface of the compact is, in some manner, sealed. The shapes are heated in a hot gas at high pressure. The gas transmits pressure to the compact because of the impervious surface. Hot isostatic pressing may take place at very high pressures for shorter times or at lower pressures for longer times. The particular approach to hot isostatic pressing depends upon the nature of the compact.
A number of techniques have been investigated for sealing the surfaces of the compacts. For example, it is sometimes possible to presinter the compact to sufficiently seal the surface of the compact prior to hot isostatic pressing. See U.S. Pat. Nos. 3,562,371 and 4,652,413. It is sometimes possible to encapsulate the compact in a flexible refractory metal casing that is evacuated and sealed around the compact prior to hot isostatic pressing. See the following patents: U.S. Pat. No. 4,230,745 (dense surface coating of a silicon metal melt), U.S. Pat. No. 4,152,223 (metallic envelope that is not removed after hipping) and U.S. Pat. No. 4,108,652 (packing presintered compact in metal powder, chloride salt and crushed refractory material and heating to form an encapsulating metal coating). Yet another technique for sealing the surface of the compact is to apply a glass or glaze to the surface that is viscous at the hot isostatic pressing temperatures. See the following patents: U.S. Pat. Nos. 4,250,610, 4,242,294, 4,199,339 and 4,104,782. For small compacts of a complex shape none of the prior art alternatives may be acceptable. Certain powders simply will not presinter sufficiently to seal the surface thereof. Metal casing is prohibitively expensive and the use of a glaze or glass may contaminate the compact with elements that are deleterious to compact properties.
Ceramic cutting tools with a substantial second phase comprising carbide whiskers have recently been introduced. Typically, these compositions are formed by hot pressing. See Wei U.S. Pat. No. 4,543,345 which teaches alumina-silicon carbide whisker compositions and teaches compaction by hot pressing. These tools are comprised of between 30 to 36 volume percent carbide whiskers. It is not possible to compact a composition with a substantial carbide whisker phase by simply presintering followed by hot isostatic pressing. See Becher and Tiegs U.S. Pat. No. 4,657,877. In some way, the whiskers prevent the adequate densification and formation of an impervious surface during the presintering.
Again, uniaxial hot pressing of whisker reinforced ceramics results in products having properties that are not all isotropic. Anisotropic properties develop as a result of the orientation of the whisker phase perpendicular to the pressing axis during pressing.